Technical Field
[0001] The present invention relates to a process for producing sorbic acid or its salt
which is useful as, for example, food additives. Specifically, the invention relates
to a process for producing sorbic acid or its salt including the step of treating
sorbic acid or its salt with activated carbon, which sorbic acid or its salt is produced
through the decomposition of a polyester obtained from ketene and crotonaldehyde.
Background Art
[0002] As processes for the commercial production of sorbic acid or its salt, a process
of reacting ketene with crotonaldehyde to yield a polyester and hydrolyzing the polyester
in the presence of an acid or an alkali, and a process of decomposing the polyester
by heat are known.
[0003] A crude sorbic acid obtained by the decomposition of the polyester generally contains
various colored substances, tar substances, and other impurities and is subjected
to purification operations such as treatment with activated carbon, distillation and
recrystallization. Particularly, the crude sorbic acid or its salt is often subjected
to treatment with activated carbon to remove colored substances.
[0004] For example, Japanese Unexamined Patent Application Publication No. 54-163516 discloses
a process for preparing a crystalline sorbic acid. This process includes the steps
of preparing a polyester from ketene and crotonaldehyde, decomposing the polyester
with hydrochloric acid in the presence of, for example, a urea compound to yield a
decomposition reaction mixture, separating the decomposition reaction mixture by filtration,
and washing the residue to yield a crude sorbic acid, adding a sodium hydroxide aqueous
solution to the crude sorbic acid to yield a sodium sorbate aqueous solution, treating
the aqueous solution with activated carbon, neutralizing and cooling the treated solution
to crystallize sorbic acid. Japanese Examined Patent Application Publication No. 44-26646
discloses a process for producing a crystalline sorbic acid. The process includes
the steps of preparing a polyester from ketene and crotonaldehyde, decomposing the
polyester with hydrochloric acid having a concentration of 35% by weight or more at
temperatures ranging from room temperature to around the boiling point of the hydrochloric
acid used, cooling the reaction mixture, separating a crude sorbic acid by filtration,
washing the crude sorbic acid with water, putting the washed crude sorbic acid into
water, heating and dissolving the sorbic acid to yield a solution, adding activated
carbon to the solution, boiling the mixture, and filtering the mixture while heating,
and gradually cooling the resulting filtrate to yield a crystalline sorbic acid.
[0005] However, according to the investigations made by the present inventors, the ability
for removing colored substances or the like contained in a crude sorbic acid greatly
varies with the type of activated carbon for use in the treatment with activated carbon.
For example, a steam-activated carbon of coconut shell origin is in wide use in the
purification of organic compounds but has a low ability for removing colored substance.
To obtain a high quality sorbic acid having a satisfactory hue by the use of this
type of activated carbon, for example, a crystallization rate in a crystallization
process subsequent to the treatment with activated carbon must be decreased or a combination
of several complicated purification operations must be employed after the treatment
with activated carbon.
Disclosure of Invention
[0006] Accordingly, an object of the invention is to provide a process for producing sorbic
acid or its salt, which is capable of efficiently removing colored substances and
is capable of easily producing a high quality sorbic acid having a satisfactory hue
in a high yield.
[0007] The present inventors made intensive investigations to achieve the above object,
and found that colored substances and other impurities can be efficiently removed
by treating a solution containing sorbic acid or its salt with a specific activated
carbon, which sorbic acid or its salt is prepared through the decomposition of the
polyester.
[0008] Specifically, the invention provides a process for producing sorbic acid or its salt
by decomposing a polyester obtained from ketene and aldehyde. The process includes
the step of treating a solution containing sorbic acid or its salt with a chemically
activated carbon, which sorbic acid or its salt is produced by the decomposition of
the polyester.
Best Mode for Carrying Out the Invention
[0009] According to the invention, a polyester obtained from ketene and crotonaldehyde is
decomposed to yield sorbic acid or its salt. The polyester is generally shown by the
following formula (1):

[0010] In the above formula, R is an acetoxy group or a hydroxyl group, and n denotes an
integer of 2 or more (e.g., about 3 to 40).
[0011] The polyester can be obtained by conventional or known processes. For example, the
polyester is obtained by reacting ketene with aldehyde in the presence of a catalyst
with or without an inert solvent. Such catalysts include, but are not limited to,
simple substances or compounds of manganese, cobalt, nickel, zinc, cadmium, and other
transition metals; and pyridine, picoline, and other nitrogen-containing basic compounds.
Examples of the compounds of the transition metals are oxides; salts of acetic acid,
salts of isobutyric acid, salts of isovaleric acid, and salts of other organic acids;
salts of sulfuric acid, salts of nitric acid, and salts of other inorganic acids;
chlorides and other halides; acetylacetone complex salts, and other complex salts
and complexes. Each of these catalysts can be used alone or in combination. The amount
of the catalyst differs according to the type of the catalyst, but is generally about
0.1 to 10% by weight relative to the weight of ketene.
[0012] The reaction of ketene with crotonaldehyde is performed at temperatures ranging from,
for example, about 20°C to 100°C, and preferably from about 25°C to 80°C.
[0013] A reaction mixture containing a polyester obtained through the reaction of ketene
with crotonaldehyde is usually distilled to remove unreacted crotonaldehyde and low
boiling impurities, and is then subjected to a decomposition reaction.
[0014] The polyester may be decomposed by hydrolysis with an acid or an alkali or by thermal
decomposition, but is preferably decomposed by hydrolysis with a mineral acid, particularly
with hydrochloric acid, for a higher yield. The polyester is hydrolyzed, for example,
at temperatures ranging from about 10°C to 110°C, and preferably from about 50°C to
100°C. An extremely low reaction temperature may invite reaction efficiency to decrease,
and in contrast, an extremely high reaction temperature may increase the by-production
of tar substances and other impurities. When the polyester is hydrolyzed with hydrochloric
acid, the concentration of hydrochloric acid is, for example, about 15 to 40% by weight,
and preferably about 23 to 36% by weight. An extremely low concentration of hydrochloric
acid may invite a decreased reaction rate, and in contrast, an extremely high concentration
of hydrochloric acid may invite disadvantages in handling property or ease of operation.
The amount of hydrochloric acid in terms of hydrogen chloride is, for example, about
10 to 160 parts by weight, and preferably about 15 to 100 parts by weight, relative
to 100 parts by weight of the polyester.
[0015] A reaction mixture obtained through the decomposition of the polyester contains colored
substances, tar substances, and other impurities by-produced in the reaction, in addition
to the sorbic acid or its salt and the catalyst used. The production of a high quality
sorbic acid or its salt therefore requires a purification process.
[0016] The invention has a main feature of including the step of treating a solution containing
the sorbic acid or its salt with a chemically activated carbon, which sorbic acid
or its salt is obtained by the decomposition of the polyester.
[0017] The treatment with activated carbon can be performed at any point in the purification
process of sorbic acid or its salt subsequent to the polyester decomposition process.
For example, when the polyester is hydrolyzed in the presence of an acid, the resulting
reaction mixture is usually a slurry containing sorbic acid dispersed in water. Generally,
the reaction mixture slurry is subjected to solid-liquid separation operation such
as filtration or centrifugal separation to yield a crude sorbic acid as a solid, and
the crude sorbic acid or its salt (e.g., a potassium salt, a sodium salt, and other
alkali metal salts), which is obtained by allowing an alkali metal hydroxide and so
on to act upon the sorbic acid, is dissolved in a solvent, and is subjected to the
treatment with activated carbon. Water is often used as the solvent for use in the
treatment with activated carbon. The water may contain a water-soluble organic solvent
such as acetone, methanol, or ethanol.
[0018] Chemically activated carbons include a wide variety of activated carbons obtained
by impregnating a material with an activating agent and firing (baking) the impregnated
material. Such materials for activated carbon include, but are not limited to, wood,
sawdust, charcoal, plain ash (carbonized sawdust), fruit shells (coconut shells, walnut
shells), fruit seeds, fruit shell coals, fruit seed coals, corn shells, peat, by-products
of pulp manufacture, lignin, sugar wastes, molasses, and other materials of plant
origin; peat, grass peat, lignite, brown coal, bituminous coal, anthracite, coke,
coal tar, coal pitch, petroleum pitch, and other materials of mineral origin; phenol
resins, vinylidene chloride resins, acrylic resins, and other synthetic resin-based
materials. Of these materials, materials of plant origin, especially, sawdust, fruit
shells, corn shells, straw of barley, peat, and other woody materials are preferred
as they are satisfactorily impregnated with an agent.
[0019] The activating agent may be any of agents having dehydration property and oxidizing
property with respect to organic compounds. Such agents include, but are not limited
to, zinc chloride; phosphoric acid; sulfuric acid; potassium hydroxide, sodium hydroxide,
potassium carbonate, sodium carbonate, sodium phosphate, calcium chloride, potassium
sulfide, potassium thiocyanate, potassium sulfate, sodium sulfate, and other alkali
metal compounds; calcium carbonate, and other alkaline earth metal compounds. As the
activating agent, acidic agents such as zinc chloride and phosphoric acid are often
used.
[0020] The specific surface area of the chemically activated carbon is generally about 200
to 3500 m
2/g, preferably about 400 to 2000 m
2/g, and more preferably about 1000 to 2000 m
2/g. The total pore volume of the chemically activated carbon is generally about 0.1
to 2 ml/g, preferably about 0.2 to 1.6 ml/g, and more preferably about 0.8 to 1.6
ml/g.
[0021] The amount of the chemically activated carbon can be appropriately selected within
a range not deteriorating the purification efficiency or other properties, and is
generally about 1 to 20 parts by weight, and preferably about 2 to 15 parts by weight
relative to 100 parts by weight of the sorbic acid or its salt to be treated with
activated carbon.
[0022] The treating temperature and pH of the solution to be treated in the treatment with
activated carbon can be appropriately selected within ranges not deteriorating the
purification efficiency and ease of operation. The treating temperature is generally
about 20°C to 100°C, preferably about 30°C to 80°C, and more preferably about 40°C
to 70°C. The pH of the solution to be treated is generally about 5 to 9, preferably
about 5.8 to 7.5, and more preferably about 6.2 to 6.8.
[0023] When the sorbic acid or its salt prepared by the decomposition of the polyester is
treated with a chemically activated carbon, colored substances can be highly efficiently
removed as compared with a treatment with a gas-activated carbon. The detail reason
of this has not been clarified, but this is probably because such a chemically activated
carbon has a pore size or pore size distribution suitable for the adsorption of colored
substances which are by-produced in the polyester decomposition process. Accordingly,
the invention can simplify the purification process subsequent to the treatment with
activated carbon and can suppress the loss of sorbic acid in the purification process
to a low level to thereby efficiently produce a high quality sorbic acid or its salt
having a satisfactory hue.
[0024] The salts of sorbic acid such as potassium sorbate and sodium sorbate can be prepared
by hydrolyzing the polyester with an alkali, or by preparing sorbic acid through,
for example, hydrolysis with an acid, and reacting the sorbic acid with, for example,
an alkali metal hydroxide to yield a salt in an appropriate step of a subsequent purification
process. The invented process can be applied to the salts of sorbic acid, as well
as to sorbic acid.
[0025] Where necessary, the sorbic acid or its salt treated with activated carbon is subjected
to a conventional separation and purification means to further improve the hue and
purity. Such separation and purification means include, for example, crystallization,
filtration, centrifugal separation, distillation, and recrystallization.
[0026] The product sorbic acid and its salts can be used as preservatives for foods such
as fish pastes, butters, cheeses, bean pastes, and jams.
[0027] The invention treats sorbic acid or its salt with a specific activated carbon, which
sorbic acid or its salt is prepared by the decomposition of the polyester, and can
efficiently remove colored substances and can easily yield a high quality sorbic acid
having a satisfactory hue in a high yield.
[0028] The present invention will now be illustrated in further detail with reference to
several inventive examples and a comparative example below, which are not intended
to limit the scope of the invention. All "parts" are by weight unless otherwise specified.
EXAMPLE 1
[0029] To 600 parts of crotonaldehyde, 2 parts of zinc isobutyrate was added as a catalyst,
and 170 parts of a ketene gas was introduced at a temperature of 30°C to 40°C to perform
a reaction. After the completion of reaction, excess crotonaldehyde was removed by
distillation under reduced pressure to yield 450 parts of a highly viscous polyester.
[0030] To 135 parts of the above-prepared polyester, 110 parts of a concentrated hydrochloric
acid having a concentration of 34% by weight was added, and the resulting mixture
was heated to 80°C to decompose the polyester to thereby yield a sorbic acid slurry.
The sorbic acid slurry was cooled and was then filtrated under reduced pressure to
yield a crude sorbic acid having a moisture content of 20% by weight, a tar content
on dry basis of 4% by weight, and a hydrochloric acid content on dry basis of 4000
ppm.
[0031] The above-prepared crude sorbic acid (65 g) and a sodium hydroxide aqueous solution
were mixed and gradually heated while stirring and adjusting pH of the resulting mixture
to 6.5. As a result, a homogenous solution was obtained at a temperature of 55°C.
To this solution, 3 g of a zinc chloride-activated carbon [trade name: CARBORAFFIN
M, material: wood flour, total pore volume: 1.4 ml/g, specific surface area: 1500
m
2/g, a product of Takeda Chemical Industries, Ltd.] was added, and the resulting mixture
was stirred for 1 hour. The mixture was then filtrated to remove the activated carbon,
and the filtrate was acidified to precipitate sorbic acid, and the precipitated sorbic
acid was filtrated and dried. In 8.8 ml of a 1 N-NaOH aqueous solution, 1 g of the
above-prepared sorbic acid was dissolved to yield a solution, and the light transmittance
of the solution at a wavelength of 400 nm was determined with a spectrophotometer.
The solution was found to have a light transmittance of 79.0%.
EXAMPLE 2
[0032] A crude sorbic acid was papered in the same manner as in Example 1. The prepared
crude sorbic acid (65 g) and a sodium hydroxide aqueous solution were mixed and gradually
heated while stirring and adjusting pH of the resulting mixture to 6.0. As a result,
a homogenous solution was obtained at a temperature of 75°C. The treatment with activated
carbon, acidification, and crystallization procedures were then performed in the same
manner as in Example 1 to yield sorbic acid. In 8.8 ml of a 1 N-NaOH aqueous solution,
1 g of the above-prepared sorbic acid was dissolved, and the light transmittance of
the solution at a wavelength of 400 nm was determined with a spectrophotometer. The
solution was found to have a light transmittance of 75.5%.
EXAMPLE 3
[0033] A crude sorbic acid was papered in the same manner as in Example 1. The prepared
crude sorbic acid (65 g) and a sodium hydroxide aqueous solution were mixed and gradually
heated while stirring and adjusting pH of the resulting mixture to 7.0. As a result,
a homogenous solution was obtained at a temperature of 35°C. The treatment with activated
carbon, acidification, and crystallization procedures were then performed in the same
manner as in Example 1 to yield sorbic acid. In 8.8 ml of a 1 N-NaOH aqueous solution,
1 g of the above-prepared sorbic acid was dissolved, and the light transmittance of
the solution at a wavelength of 400 nm was determined with a spectrophotometer. The
solution was found to have a light transmittance of 75.6%.
EXAMPLE 4
[0034] Sorbic acid was prepared in the same manner as in Example 1, except that a zinc chloride-activated
carbon [trade name: ZN-D, material: wood flour, total pore volume: 1.3 ml/g, specific
surface area: 1400 m
2/g, a product of Hokuetsu Carbon Industry Co., Ltd.] was used as the activated carbon.
In 8.8 ml of a 1 N-NaOH aqueous solution, 1 g of the above-prepared sorbic acid was
dissolved, and the light transmittance of the solution at a wavelength of 400 nm was
determined with a spectrophotometer. The solution was found to have a light transmittance
of 72.2%.
COMPARATIVE EXAMPLE 1
[0035] Sorbic acid was prepared in the same manner as in Example 1, except that a steam-activated
carbon [trade name: NISSO-MERSAN EEP-02, material: coconut shell, total pore volume:
0.3 ml/g, specific surface area: 500 m
2/g, a product of Nippon Soda Co., Ltd.] was used as the activated carbon. In 8.8 ml
of a 1 N-NaOH aqueous solution, 1 g of the above-prepared sorbic acid was dissolved,
and the light transmittance of the solution at a wavelength of 400 nm was determined
with a spectrophotometer. The solution was found to have a light transmittance of
55.4%.